Improved hydrocarbon compositions are needed to help meet the growing demand for middle distillate products, such as aviation turbine fuels, for example, JP-8, and diesel fuel. Diesel fuel generally provides a higher energy efficiency in compression ignition engines than automotive gasoline provides in spark combustion engines, and has a higher rate of demand growth than automotive gasoline, especially outside the U.S. Further, improved fuel compositions are needed to meet the stringent quality specifications for aviation fuel and the ever tightening quality specifications for diesel fuel as established by industry requirements and governmental regulations.
One known route for producing hydrocarbon compositions useful as fuels is the oligomerization of olefins over various molecular sieve catalysts. Exemplary patents relating to olefin oligomerization include U.S. Pat. Nos. 4,444,988; 4,456,781; 4,504,693; 4,547,612 and 4,879,428. In these disclosures, feedstock olefins are mixed with an olefinic recycle material and contacted with a zeolite, particularly in a series of fixed bed reactors. The oligomerized reaction product is then separated to provide a distillate stream, and typically a gasoline stream, and any number of olefinic recycle streams.
However, in these known oligomerization processes, the focus is on producing relatively heavy distillate products, and even lube base stocks. To enable the production of relatively heavy materials, the processes employ, either directly or indirectly, a relatively large amount of olefinic recycle (typically >2:1 w/w relative to feed), containing significant quantities of C10+ material. The relatively large recycle rate provides control over the exotherm of the oligomerization reaction in the preferred fixed bed, adiabatic reactor system, while the relatively heavy recycle composition (in conjunction with high conversion of light olefin feed, in part enabled by a relatively low WHSV) enables the growth of heavier oligomers and thus higher molecular weight and denser distillate product. However, the high rate of recycle requires much larger equipment to handle the increased volumetric flow rate, and uses more separation/fractionation energy, and hence more and larger associated energy conservation elements. Further, the high molecular weight of the oligomer product requires very high temperatures for the fractionation tower bottoms streams that may eliminate the use of simple steam reboilers and require more expensive and complicated fired heaters.
The recycle streams in conventional olefin oligomerization processes are produced in a variety of fashions, typically including some sort of single stage flash drum providing a very crude separation of reactor product as a means of providing the relatively heavy components, followed by various fractionation schemes which may or may not provide sharper separations, and again often provide heavy components as recycle. The dense distillate product is generally characterized by a relatively high specific gravity (in excess of 0.775) and a high viscosity, in part due to the composition comprising relatively high levels of aromatics and naphthenes.
Very few references discuss both the merits and methods of producing lighter distillate products, typified by such as jet fuel, kerosene and No. 1 Diesel, via the oligomerization of C3 to C8 olefins. Jet/kero is generally overlooked as a particularly useful middle distillate product, inasmuch as the volume consumed in the marketplace is considerably smaller than its heavier cousins, No. 2 Diesel and No. 4 Diesel (fuel oil). However, jet/kero is a high volume commercial product in its own right, and is also typically suitable as a particular light grade of diesel, called No. 1 Diesel, that is especially useful in colder climates given its tendency to remain liquid and sustain volatility at much lower temperatures. In addition, jet/kero type streams are often blended in with other stocks to produce No. 2 Diesel, both to modify the diesel fuel characteristics, and to allow introduction of otherwise less valuable blendstocks into the final higher value product.
U.S. Pat. No. 4,720,600 discloses an oligomerization process for converting lower olefins to distillate hydrocarbons, especially useful as high quality jet or diesel fuels, wherein an olefinic feedstock is reacted over a shape selective acid zeolite, such as ZSM-5, to oligomerize feedstock olefins and further convert recycled hydrocarbons. The reactor effluent is fractionated to recover a light-middle distillate range product stream and to obtain light and heavy hydrocarbon streams for recycle. The middle distillate product has a boiling range of about 165° C. to 290° C. and contains substantially linear C9 to C16 mono-olefinic hydrocarbons, whereas the major portion of the C6 to C8 hydrocarbon components are contained in the lower boiling recycle stream, and the major portion (e.g. 50 wt. % to more than 90 wt. %) of the C16+ hydrocarbon components are contained in the heavy recycle fraction.
U.S. Pat. No. 4,788,366 discloses a multi-stage process for upgrading an ethene-rich feed into heavier hydrocarbon products boiling in the lubricant, distillate and gasoline ranges. The process involves initially contacting the ethene-rich feed in a primary reaction stage with a fluidized bed of a zeolite catalyst, such as ZSM-5, and then separating the resultant effluent into at least a liquid stream containing a major amount of aromatics-rich C5+ hydrocarbons and a gas stream rich in propene and butene. The gas stream is then fed to a secondary reaction stage comprising a series of fixed bed reactors containing a medium pore zeolite oligomerization catalyst, such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23 or ZSM-35, preferably having a silica/alumina molar ratio of 20 to 200 and a crystal size of 0.2 to 1 micron. In the secondary reaction stage, at least part of the aromatics-rich, liquid primary stage effluent is mixed with a hot inter-stage stream containing partially upgraded olefins to quench said inter-stage stream and the resultant mixed stream is passed to at least one downstream oligomerization reactor. The conditions in the secondary reaction stage can be varied to control the product slate, but generally include a temperature of 235° C. to 315° C., a pressure of 2800 to 10,000 kPa and a weight hourly space velocity of 0.1 to 1.5. The product necessarily contains a significant quantity of aromatic hydrocarbons.
A similar process is described in U.S. Pat. No. 4,855,524, in which an olefin-containing light gas or light naphtha is oligomerized to a C10+ aliphatic hydrocarbon product in multistage reaction zones. In particular, lower alkenes in the feed are oligomerized to intermediate range olefins, mainly in the C5 to C9 range, in a low severity primary reaction zone containing zeolite catalyst particles, preferably in the form of a fluidized bed. The primary reaction zone effluent is then separated into a C4− light gas stream and a predominantly olefinic C5+ intermediate stream substantially free of C4− components. The intermediate stream is then contacted with a medium pore, shape selective, acid oligomerization catalyst in a secondary reaction zone under oligomerization conditions to produce a predominantly C10+ product. To maximize the yield of distillate product, the '524 patent teaches that C10+ hydrocarbons should be removed from said intermediate stream before passage through said secondary reaction zone and that said secondary reaction zone should be operated with catalyst having an average activity alpha greater than 10, at weight hourly space velocity (WHSV) in the range from about 0.1 to about 10 hr−1, at an inlet pressure in excess of about 3200 kPa, an inlet temperature in the range from about 149° C. to about 232° C. and an outlet temperature in the range from about 232° C. to about 343° C. The overall yield and/or quality of the distillate may be further increased by recycling an insufficiently oligomerized portion of the product stream to the secondary reaction zone.
In accordance with the known olefin conversion and oligomerization processes, catalysts are specified that have certain characteristics conducive to their desired products, typically aromatics and heavier distillate products, even lube base stocks. Such characteristics of these known catalysts are not necessarily conducive to the production of lighter distillate products, for example, relatively large crystal size to constrain the larger molecules to enable oligomerization, and relatively high activity to increase the rate of reaction of the less reactive larger molecules. Further, such catalyst attributes in conjunction with the known process conditions favor the production of byproduct cyclics, e.g., aromatics, which are known to be detrimental to distillate and aviation fuel properties.
According to the present invention, it has now been found that by controlling the conditions of the oligomerization process and, in particular, the amount and composition of the recycle, C3 to C8 olefins can be converted into a novel hydrocarbon composition similar in make-up to that of conventional diesel and jet fuel, but with an unusually low specific gravity making it an excellent blending stock to produce fuel products, such as Jet Fuel A and No. 1 and No. 2 Diesel. In addition, the hydrocarbon composition of the invention is very low in sulfur, naphthenes and aromatics, has a high cetane number and, in view of its low n-paraffin content, has a very low freezing point.